Tap and the othes classes in tap.dart are not really related.

We don't want to expose these from the gesture package. This patch moves
DoubleTapGestureRecognizer into tap.dart so we can make TapTracker and
TapGesture private to that file.

I've also removed the top-level description of the Sky package. Instead, we
should host that content on flutter.io.

This patch is part of a sequence of patches towards fewer top-level libraries.
In this patch, the gesture libraries are combined into one gestures.dart
library.